Last July, a small Canadian environmental group called Windsor on Watch captured footage of a frightening spectacle — a giant black cloud, dark and ominous as a thunderhead, swirling into Ontario from neighboring Detroit.

The cloud, it turned out, was dust from piles of petroleum coke, or petcoke, a byproduct of refining oil. The petcoke was the property of Koch Carbon, a company controlled by the Koch brothers, who have made much of their $100 billion fortune from fossil fuels. Though the dirty piles were eventually removed from the banks of the Detroit River, their influence is still being felt in Michigan's US Senate race, which pits Democrat Gary Peters against Republican Terri Lynn Land.

Although climate change has loomed large in a number of national races this election season, environmental issues have been exceptionally prominent in the Great Lakes State. In recent years, the state has experienced crises including a major oil pipeline spill along the Kalamazoo River and toxic algal blooms on Lake Erie that contaminated drinking water.

"If you had to name an issue that has dominated this race, it would be climate change, energy, and the environment," Susan Demas, publisher and editor of Inside Michigan Politics, told VICE News. "It's more of an issue here than in other states."

That fact is underscored by last summer's petcoke incident. Soon after the story broke, Peters, who has served in the U.S. House of Representatives since 2009, held a press conference in front of the black mounds. Calling petcoke "dirtier than the dirtiest coal," Peters announced that he would introduce legislation to study the impacts of the substance on human health. The piles were removed over the winter, a step that many environmentalists attribute to Peters' advocacy.

For conservation groups, the episode furthered the green credentials of Peters, who owns a lifetime score of 89 percent on the League of Conservation Voters' National Environmental Scorecard.

"He was the one who led the charge on the petcoke piles," Heather Taylor-Miesle, director of the NRDC Action Fund, told VICE News. "He's one of the only members of Congress who's taken on the Koch brothers directly."

That distinction, Taylor-Miesle added, is especially striking given that Land's supporters include the conservative Americans for Prosperity, the PAC founded by Charles and David Koch. In Michigan, AFP has poured $2 million into over 4,500 political ads.

Like many Koch-linked Republicans, Land, who served as Michigan's Secretary of State from 2003 to 2010, has taken fire from environmental groups for her climate beliefs. Though she expressed skepticism early in her campaign that humans were impacting the climate to a large extent, she later switched course, insisting via Twitter that climate change was "absolutely real." However, she went on to say that her top priority was employment, and that climate change regulation would cost Michigan jobs.

"Her views are clearly radical, and out of touch with what people in Michigan and across the country want," Tiernan Sittenfeld, LCV's Senior Vice President of Government Affairs, told VICE News. "It's not difficult to make the case that she's not fit to be the Senator from Michigan."

Peters, too, has benefited from outside money. His supporters include NextGen Climate Action, the PAC operated by former hedge fund manager Tom Steyer, who has pledged to spend $100 million to defeat candidates who oppose action on climate change. Steyer's group has spent over $3.6 million campaigning against Land.

An ad from Crossroads GPS, the PAC operated by Republican consultant Karl Rove, criticized Peters' opposition to the Keystone XL pipeline, asserting that Steyer would profit if the pipeline was blocked — a claim that is outdated because Steyer has sold his investments in rival pipeline companies.

Though Peters has made his opposition to petcoke a significant plank in his platform, his record on the issue isn't spotless. This summer, he elicited charges of hypocrisy for his $19,000 personal investment in the French energy company Total S.A., which owns a petcoke facility in Port Arthur, Texas.

He also raised eyebrows in June, when he expressed some reservations over a new Environmental Protection Agency rule — much hailed by environmentalists — that would curb carbon emissions from existing power plants. Although Peters voiced general support for the rule, he said he feared it would impose "a more stringent standard on Michigan than surrounding Midwestern states."

Taylor-Miesle said Peters just wants to make sure that Michigan gets full credit from the EPA for the clean energy upgrades it's already made.

"That seems to be a pretty reasonable request," Taylor-Miesle told VICE News. "He's not going to be a rubber stamp, but folks who are rubber stamps aren't the ones who make the deals."

"This is certainly Gary Peters' race to lose, and might be one of the few bright spots for Democrats on Election Day," Susan Demas said.

Recent polls show Peters ahead by double-digits. Such a sizable edge, Demas told VICE News, would have been "unthinkable" back in the spring, when Land led many polls.

Demas attributes the turnaround mostly to Peters' smart, safe campaign and Land's failure to connect with voters. But for Peters' green backers, Michigan is proof that environmental issues can be winning ones.

Sittenfeld said 79 percent of all pro-Peters campaign ads have focused on the environment.

"This race illustrates the extent to which talking about clean energy and the climate crisis is not only good policy, but good politics," she told VICE News.